Simple Low Voltage Tube Preamp?

CarlosT · 30th January 2007, 05:42 PM

Have never done anything with tubes but I am intrigued about the "sound" of tubes. I was thinking about a simple tube pre-amp to an LM3875 chipamp.

I came across this low-voltage tube 512AX

CK512AX

Pinout: 1(red dot)=anode, 2=screen, 3=filament, 4=g1, 5=filament Vf=0.625V, If=20mA. Va max=45V, Vsg=45V. No power limitation, since you can't draw enough current to do anything bad.

Pentode mode: gm=160uS. Triode mode:gm=250uS, Ra=35k, MU=9.

Reasonable amplifier bias points:

Pentode: 0.7V bias. Va=22.5V, Vsg=27V, Anode supply=45V. anode resistor=180k. Ia=.12mA, Isg=.06mA.

Triode: 0.6V bias. Va+sg=22.5V, Anode supply=45V, load resistor=100k, Ia+sg=.2mA.

Could this be simply implemented with 25V transformers? Are there other simple miniature or low-voltage tubes that would sound good? Since this would be a preamp, a lotta gain is not important or even preferred.

Thanks!

kevinkr · 30th January 2007, 06:12 PM

Steve Bench designed and built a battery powered pre-amp based on low voltage sub mini tubes. I think your proposal will work ok with 25V transformers. Another good low voltage candidate incidentally is the 6GM8.

Jaime · 30th January 2007, 07:18 PM

Wavelength Audio (Gordon Rankin) made an expensive preamplifier with the 6GM8 tube. I think is a good option.

http://www.wavelengthaudio.com/preamps.html

best regards Jaime

MRupp · 30th January 2007, 08:16 PM

OK, maybe not quite as simple, but you can build an Aikido preamp with the 6GM8 / ECC86, B+ voltage can be as low as 24V: http://www.tubecad.com/2006/04/blog0059.htm

As a benefit you can get superb PCBs from John Broskie at a reasonable price.

Audio_idiot · 31st January 2007, 08:40 AM

Hi CarlosT,
What is your intention of the tube pre? do you need some gain or just a tube buffer?

Ecc88, 6922 can do at fairly low plate voltage of about 90V, low enough Rp to give low enough Zo, with a suitable plate choke, I think you can use ~100V B+ for the whole stage.

6AS7 or 6080 can also work in the 40~60 B+ zone... cathode follower if you must have that super low output mpedence, even with normal ground cathode, the impedance would still low enough for most applications.

6GM8 is good candidate, but From my experiments, I think the current is too small to produce good sounding cathode follower... I've not tried other method...

Cheers

CarlosT · 31st January 2007, 09:57 PM

WOW! So much to learn...so little time

That Aikido stuff was cool...as was the blog.

I got a buncha 25V + 25V toroidal trannies around so that's why I was thinking about low voltage tubes. I also currently have a combined channel mono LM3875 design which coulda benefitted from a buffer.

davidlzimmer · 31st January 2007, 10:22 PM

Dang you guys!

This stuff is SOOO tempting!

Could someone elaborate on the two outputs of the Aikido pre? Could a similar circuit be used with just one tube per channel?

A schematic for mm phono also would be great.

Thinking about building one as a gift to my brother in law.

Thanks!

davidlzimmer · 31st January 2007, 11:06 PM

Oops! Just realized you need 2 tubes to total 24v for the filaments.

Sheldon · 1st February 2007, 12:21 AM

Quote:

Originally posted by davidlzimmer
Dang you guys! This stuff is SOOO tempting!

Could someone elaborate on the two outputs of the Aikido pre? Could a similar circuit be used with just one tube per channel?

A schematic for mm phono also would be great.

Thinking about building one as a gift to my brother in law.

Thanks!

Not one tube per channel, but you can use a single section on the input side - so three dual triodes total for two channels.

http://www.tubecad.com/2004/blog0013.htm

Sheldon

davidlzimmer · 1st February 2007, 01:32 AM

Sheldon,

Thanks, but I was wanting to stick with the 6GM8.

More importantly, the amp doesn't really need a pre. But an RIAA pre would be sweet!

30th January 2007, 05:42 PM	#1
CarlosT diyAudio Member Join Date: Oct 2006	Simple Low Voltage Tube Preamp? Have never done anything with tubes but I am intrigued about the "sound" of tubes. I was thinking about a simple tube pre-amp to an LM3875 chipamp. I came across this low-voltage tube 512AX CK512AX Pinout: 1(red dot)=anode, 2=screen, 3=filament, 4=g1, 5=filament Vf=0.625V, If=20mA. Va max=45V, Vsg=45V. No power limitation, since you can't draw enough current to do anything bad. Pentode mode: gm=160uS. Triode mode:gm=250uS, Ra=35k, MU=9. Reasonable amplifier bias points: Pentode: 0.7V bias. Va=22.5V, Vsg=27V, Anode supply=45V. anode resistor=180k. Ia=.12mA, Isg=.06mA. Triode: 0.6V bias. Va+sg=22.5V, Anode supply=45V, load resistor=100k, Ia+sg=.2mA. Could this be simply implemented with 25V transformers? Are there other simple miniature or low-voltage tubes that would sound good? Since this would be a preamp, a lotta gain is not important or even preferred. Thanks!

30th January 2007, 06:12 PM	#2
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	Steve Bench designed and built a battery powered pre-amp based on low voltage sub mini tubes. I think your proposal will work ok with 25V transformers. Another good low voltage candidate incidentally is the 6GM8. __________________ www.kta-hifi.net

30th January 2007, 07:18 PM	#3
Jaime diyAudio Member Join Date: Apr 2004 Location: Montevideo	Wavelength Audio (Gordon Rankin) made an expensive preamplifier with the 6GM8 tube. I think is a good option. http://www.wavelengthaudio.com/preamps.html best regards Jaime __________________ I write in English worse than Tarzan

30th January 2007, 08:16 PM	#4
MRupp diyAudio Member Join Date: Dec 2003 Location: Frankfurt area	Low Voltage Aikido preamp OK, maybe not quite as simple, but you can build an Aikido preamp with the 6GM8 / ECC86, B+ voltage can be as low as 24V: http://www.tubecad.com/2006/04/blog0059.htm As a benefit you can get superb PCBs from John Broskie at a reasonable price. __________________ Martin Rupp

31st January 2007, 08:40 AM	#5
Audio_idiot diyAudio Member Join Date: Feb 2006 Location: Malaysia	Hi CarlosT, What is your intention of the tube pre? do you need some gain or just a tube buffer? Ecc88, 6922 can do at fairly low plate voltage of about 90V, low enough Rp to give low enough Zo, with a suitable plate choke, I think you can use ~100V B+ for the whole stage. 6AS7 or 6080 can also work in the 40~60 B+ zone... cathode follower if you must have that super low output mpedence, even with normal ground cathode, the impedance would still low enough for most applications. 6GM8 is good candidate, but From my experiments, I think the current is too small to produce good sounding cathode follower... I've not tried other method... Cheers __________________ Audio_Idiot

31st January 2007, 09:57 PM	#6
CarlosT diyAudio Member Join Date: Oct 2006	WOW! So much to learn...so little time That Aikido stuff was cool...as was the blog. I got a buncha 25V + 25V toroidal trannies around so that's why I was thinking about low voltage tubes. I also currently have a combined channel mono LM3875 design which coulda benefitted from a buffer.

31st January 2007, 10:22 PM	#7
davidlzimmer diyAudio Member Join Date: Jan 2006 Location: Fairmount, GA	Dang you guys! This stuff is SOOO tempting! Could someone elaborate on the two outputs of the Aikido pre? Could a similar circuit be used with just one tube per channel? A schematic for mm phono also would be great. Thinking about building one as a gift to my brother in law. Thanks! __________________ Poor stereo mix? Switch to mono! Perfect.

31st January 2007, 11:06 PM	#8
davidlzimmer diyAudio Member Join Date: Jan 2006 Location: Fairmount, GA	Oops! Just realized you need 2 tubes to total 24v for the filaments. __________________ Poor stereo mix? Switch to mono! Perfect.

1st February 2007, 01:32 AM	#10
davidlzimmer diyAudio Member Join Date: Jan 2006 Location: Fairmount, GA	Sheldon, Thanks, but I was wanting to stick with the 6GM8. More importantly, the amp doesn't really need a pre. But an RIAA pre would be sweet! __________________ Poor stereo mix? Switch to mono! Perfect.

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